Hidden from the road by a shroud of scaffolding, the partially completed terrace of three houses looks much like any other. But step inside and the difference in their construction is immediately apparent – lightweight concrete blocks and panels have been used everywhere. When they are complete in May, these will be the first houses in the UK to have a structure constructed entirely from aircrete maker H+H Celcon's Jämerä system. The building's walls and foundations are constructed from aircrete blocks, the floors and roof are formed from aircrete planks even the lintels spanning the doors and windows are – you guessed it – aircrete.
Aircrete is made of a mixture of materials including ground sand, pulverised fuel ash cement, lime, aluminium powder and water. These are blended together into a mixture that foams, trapping air in the cementitious matrix. This is then cut into sections before it fully hardens, to create solid, light, strong blocks, which, because of their air bubbles, also have good insulating properties. The system is not new; it has been used in Scandinavia for almost 20 years. However, Celcon has adapted the Scandinavian version for the UK market and is about to roll out the complete system in the UK.
Victoria Park Housing Association, the owner of the plot, was looking to trial different construction technologies on this site. The association had originally planned to pioneer an American steel frame system but "negotiations broke down", according to Peter Marten, development director of Laing Partnership, the social housing arm of the Laing Homes Group. "We were keen to find an alternative to timber frame," explains Marten, "because there is still resistance to the use of timber frame by some registered social landlords." Marten says Jämerä offered an alternative form of construction for this diminutive site, and it is expected to cost the same as using traditional brick-and-block masonry.
To measure the success of the system, Laing Partnership is to benchmark the project. The housebuilder has an innovation evaluation group, but for this project they also involved BRE, which is undertaking a government-funded study on sustainability and prefabrication. However, BRE wanted a similar brick-and-block-built terrace for the purposes of comparison. "Laing Partnership doesn't tend to build three-unit schemes," explains Marten. "So we had to identify three similar units on a much larger site in Edmonton to use as a comparator."
Because Laing Partnership was looking to roll out the system on other schemes, it involved Tower Hamlets Building Control and Housing Association Property Mutual – the building defects insurer for RSLs. "These bodies help give the project independent verification," Marten explains. And the housing association was also able to secure innovation funding from the social housing funder the Housing Corporation.
To eliminate the differences in ground conditions, both projects are being timed from the ground floor upward. Below ground on the Jämerä site, the building's walls are constructed from 350 mm wide, 240 mm deep and 430 mm long blocks. The ground floor panels sit on top of these walls, separated from them by a dampproof course. "Using aircrete for both the ground floor and the foundations means that additional insulation is not needed to comply with the requirements of the new Part L," says Ian Exall, marketing manager at H+H Celcon.
The ground floor is constructed from slabs of reinforced aircrete. These elements are 600 mm wide, 250 mm deep and up to 6 m long, and slot together using a tongue-and-groove system. The planks are craned straight from the delivery truck into position. Once in place, grout binds and seals the panels together. Manufacturing tolerances mean the floor panels are relatively flat. "They do not need a big floor screed, just a thin finishing and levelling screed," Exall explains. In this scheme, the panels used to construct the ground floor were also used for the floor and ceiling of the intermediate floor.
As with the foundations, the external walls are constructed from a single skin of the same 350 mm wide blocks. The blocks are assembled using Celcon's thin-joint mortar system, which uses a 3 mm mortar joint between the blocks rather than the more conventional 10 mm thick trowel-applied mortar. The disadvantage of the system is that there is little scope for correcting inconsistencies. "The key to the wall construction is to ensure the first course of blocks is absolutely level," says Marten. Here, the first course of blocks is bedded on a 10 mm thick layer of mortar.
The wall's single-skin construction means it can be built very quickly. Exall says: "This is down to a combination of not having to construct a cavity and the quick-setting qualities of the thin joint mortar." Originally, the intention was to construct the walls from 300 mm thick blocks, but for the walls to comply with energy regulations their thickness had to be increased to 350 mm, giving the walls a U-value of 0.32 W/m2°C.
For this project, the reinforced aircrete lintels that span the doors and windows had to be imported from Celcon's Finland plant. The units are fabricated from the same material as the walls, which eliminates the problem of cold bridging. Unfortunately, these elements were not fabricated to the same height as the blocks used to construct the walls, so aircrete packing pieces had to be inserted beneath the lintels. Once Celcon's UK factory starts to produce lintels later this year, such inconsistency will not be a problem.
On site, the houses are almost complete. A crew of bricklayers is adding the final few blocks to the gables, ready the for installation of the aircrete roof panels. Once again, these will be the same as those used to construct the floors. The panels are slotted together lengthways to span between the gables, eliminating the need for a trussed rafter system and allowing an additional room in the roof to be created. Counter battens and tiles will then be added to give the roof a traditional appearance, and an additional 50 mm thick quilt of insulation inserted between the battens will ensure the house is sufficiently insulated to meet the new Part L.
The system has certainly found favour with the team on site. "I work for myself and the rain we've had this week would normally mean that I'd have to stop work," says Rob Peters, contracts manager at Steve Murphy Brickwork. However, the thin joint system means that work can continue whatever the weather. In fact the aircrete system has already proved itself to be faster than the traditional methods used on the comparator project: so much so that the Jämerä team was forced to stop work to allow the Edmonton project to catch up.
Another advantage – as Peters is quick to point out – is that, aside from himself, there is only one other skilled bricklayer on the site – "to ensure the walls are set out correctly". The simplicity of the system means that it is being assembled by an apprentice and a couple of trainees. And because there are no carpenters on site, the same team that built the walls has assembled the floors and roof structure.
The measurement of the comparison between this site and the one in Edmonton, which was started on 14 January, will stop once the finishes have been applied. The smooth wall finish created by the thin joint mortar system means that a thin spray coating of plaster is all it takes to finish the interior. The only preparation needed prior to the application of plaster is to run a rasp over any projecting areas of mortar. Externally, Laing Partnership will be testing a combination of three different finishes: untreated hardwood, the "Corium" brick cladding system, and spray-applied unplasticised render. However, the housebuilder could have chosen any finish for outside of the dwellings, as the single-skin walls already comply with energy regulations.
Laing Partnership is optimistic that the Jämerä scheme will pass the comparison test with flying colours. In fact, the firm is looking to use the system on blocks of flats and apartments. For now, though, the building's new tenants must wait for the builders to finish. And who knows? By then Celcon may even have launched a range of aircrete furniture to fill their aircrete houses.